The present invention relates to aerosol inhalation devices for application of medication to the respiratory tract.
Aerosol inhalation devices are primarily used for local application of medications to the lower parts of the respiratory tract. Such local administration of medication results in rapid response to the medication with a low dosage as compared to systematic administration of the same drug.
Aerosol devices for administration of medication must be designed to have suitable spraying properties, which are reproducible during the useful lifetime of the inhalation device. In order to achieve uniform spraying properties, it is neccessary for the propellant pressure of the device to remain constant. The propellant pressure should be also selected to be adequate to produce a fine spray. If spray particles are excessively large, they will be deposited primarily in the oral cavity or upper respiratory tract.
Conventional inhalation devices, such as described in U.S. Pat. Nos. 3,001,524 and 3,012,555 make use of a comparatively large container having both medication and propellant, which is permanently mounted to a dispensing unit. The medication is either dissolved or suspended in the propellant which is typically a chlorofluoroalkane, such as Freon..RTM. Such fluorocarbon-type propellants have a pressure of 0.3 to 0.5 X10.sup.6 Pascal (3000 to 5000 gm/cm.sup.2) at room temperature and have previously been considered to have low toxicity. Fluorocarbon propellants have therefore gained wide use in inhalation devices as well as many other aerosols. Recently, the toxicity of fluorocarbon propellants has been questioned, and recommendations have been made to use substitute propellants in medical inhalation devices (see Toxicology 3 (1975), pp. 321-332). In addition, some scientists have raised questions regarding the possibility that fluorocarbon propellants contribute to the depletion of the atmospheric ozone layer.
Prior inhalation devices have further disadvantages since the permanent mounting of the dispensing apparatus to the medication container results in the disposal of the rather expensive dispensing apparatus when the container is empty. Further, the available pressure in these units is limited by the container to no more than 0.5 X10.sup.6 Pascals. (5000 gm/cm.sup.2) This is a distinct disadvantage, since higher propellant pressure can produce in a more finely divided aerosol spray, resulting in more rapid propellant evaporation and more uniform medication distribution within the respiratory tract. Another problem with prior aerosol devices is that they are susceptible to excess use by the patient. These devices permit medication to be applied with each breath and contain sufficient medication for approximately 200-400 inhalations. The patient may therefore tend to apply more medication than is appropriate.
While it is evident that it is desirable in an inhalation device to use a non-toxic propellant, such as carbon dioxide, prior art aerosol mechanisms using carbon dioxide as a propellant are not appropriate for use in inhalation devices. Prior art carbon dioxide propelled mechanisms make use of dissolved carbon dioxide to produce spraying pressure. This is unsuitable for an aerosol inhalation device because the propellant pressure decreases as the contents of the container are reduced. Such decrease in propellant pressure reduces the ability of the inhalation device to apply medication to the lower respiratory tract.
It is therefore an object of the present invention to provide an aerosol inhalation device capable of using propellants other than fluorocarbons.
It is a further object of the invention to provide such a device with uniform aerosol properties.
It is a still further object of the invention to provide such a device which does not lend itself to excess use by the patient.